Keying circuit



Oct. 15, 1940. H.-J. H. WASSELL 2,

KEYING C-IRCUIT Filed Nov. 16, 1938 INVENTOR HAROLD JOSEPH HUGHES MISSEU,

Patented Oct. 15, 1940 KEYING cmom'r Harold Joseph Hughes Wassell, Widford Grove, Chelmsford, England; assignor to Radio Corporation of America, a corporation of Delaware Application November 1c, 1938, Serial no. 240,641

- In Great Britain December 1, 1937 I t 3 Claims. (Cl. 178-66) This invention relates to keying circuit arrangements for radio and like transmitters and has for its object to provide improved keying circuits for radio and the like transmitters of the kind wherein there isa plurality of high frequency stages in cascade one or more of said stages being keyed in and out of the cut-off condition thereby to remove the high frequency drive from a succeeding stage or stages and one or more of the succeeding stages being simultaneously keyed in and out of a condition in which considerable dead loss current is taken, thereby improving the regulation since, when the high frequency drive is cut oil by an early stage or stages, the total load upon the energizing power source is still maintained at a value more or less approximating to, or a considerable percentage of, that obtaining prior to such out off, by the action of the dead loss tube or tubes. By the term dead loss current, applicant means power loss in the anode circuit of a valve under directcurrent conditions. In most known transmitters of the kind referred to the necessary changes in the operating conditions of the stages are brought about by suitably changing the grid biasses upon the stages in question, the usual practice being to derive the biasses from a resistance which is in series with a keying tube and suitable anode voltage supply source therefor, the grid potential of the keying tube being controlled by a key or the like which accordingly controls the current flow through the said resistance and therefore the biasses applied to the high frequency stages. Such a known circuit arrangement offers the practical defect that the said resistance must be of low value as compared to the grid 'impedances of the high frequency stage since high frequency stage grid currents flowing in the mark condition must flow through the said resistance. The necessity for a low 'value of resistance in turn involves a necessity for a high current flow therethrough to develop the required bias potentials therein and this in its turn involves the use of a large and expensive keying tube and associated anode potential source. The principal object of this invention is to avoid the above defect.

According to this invention a keying circuit for a radio or like transmitter of the kind referred to and wherein the two required keying biasses for the high frequency tubesnamely the cut-off bias for one stage or stages and the dead loss bias for a later stage or stages-are obtained from a potentiometer resistance in circuit with a I keying tube is characterized in that there are also provided two auxiliary tube stages connected to receive controljbiasses from said resistance one of 1 said auxiliary tube stages being shuntedbetwee'n the point of application of one of said keying biasses and the end ofsaid resistance and the other of said auxiliaryitubes being shunted between the point of application of the other of said keying biasses and said resistance and the biasses applied to said auxiliary tube stages, in keying, being such as to render them conductive when grid currents are occasioned in the appropriate high frequency stages. When gridcurrent flows in the high frequency stages my invention. provides'two paths through which it may flow, i. e., the additional tube impedances and the said resistance. As a consequence the resistance, keying tube and plate supply source,

- therefore, may be smaller.

The invention is'illustrated in the accompanying drawing which shows diagrammatically one embodiment of the invention as applied toa radio transmitter having five high frequency stages-represented by the rectangles I, 2, 3, 4,

5,in cascade, of which the second (2), third (3),

and fourth (4) are keyed together into and out of the non-conductive condition and the fifth (5) is simultaneously keyed into and out of the dead loss condition. It will, of course, be appreciated that the invention is not limited to its application to transmitters with this particular number and arrangement of stages.

Referring. more in detail to the figure the grid circuits of the second, third and fourth stages '2, 3, 4 are connected to the cathode 6 of a first auxiliary tube T whose anode 8 is connected to earth and to the cathode 9 of a keying tube Ill. The grid circuit of the fifth stage 5 is connected to the cathode ll of a second auxiliary tube I2 whose anode I3 is also connected to the cathode 9 of the keyingtube III. A bias source I in series with a key l5 (or relay contacts) is connected in the grid-cathode circuit of the keying tube I 0 whose anode-cathode circuit consists of an anode potential source IS in series with a potentiometer resistance l'l. Any other keying means well known per se may, of course, replace the bias source and key or relay contacts in the grid-cathode circuit of the keying tube "I. The end [8. (or a point near the end) of the resistance l1 adjacent the negative terminal of the anode potential source 16, is connected to the control grid l9 of the first auxiliary tube 1 the cathode 6 of. which is connected to a point 20 further down on the resistance H. The control grid 2! of the second auxiliary tube 12 is connected to a point 22 near the earthed end of the resistance l1 and the cathode H of said auxiliary tube I2 is connected to a point 21 still nearer said earthed end. If desired, and preferably, all the points I8, 20, 22, 23 on the potentiometer resistance l1 may be made adjustable as indicated.

In the mark" condition (when the keying tube II is not conductive and accordingly the grids of the auxiliary tubes 1, ii are at approximately the same potentials as the associated cathodes) the said tubes present low internal resistances and thus allow the major portion of the grid current from the high frequency stages'to flow throu them in preference to flowing through the relatively high potentiometer resistance H. The positions of the taps I8, 20, 22, 23 are so chosen that at.spaee" (when the keying tube l becomes nductive), the auxiliary tubes are biassed to Rt oil and at the same time the cathodes of said tubes assume the potentials necessary for the (second, third and fourth stages 2, I, l, to swing to cut-off and the fifth stage 5, to dead loss.

As will now resistance II can be made of quite high value, while the keying tube Hi can be quite small and the anode potential source [6 therefor also small, e. g. capable of supplying only a few milliamperes.

What is claimed is: I

1. In a keying system, a plurality of relay stages each comprising a tube having a control grid and cathode, a source of potential and a resistance connected in a series circuit, means connecting points on said resistance between the cathode and control grid of at least one of said tubes, an additional tube having its impedance connected between points on said resistance, to shunt a portion of said resistance connected between said control grid and cathode, an impedance variable at signal frequency in shunt to said series circuit, said variable impedance serving to vary the potential drop in said resistance and the bias between said control grid and cathode to grid of said additional be appreciated the potentiometer render the said first tube conductive or non-com.

and a connection between the control tube and said resistance at a point on said resistance which biasses said additional tube to a point at which its impedance is low when said first tube is conductive.

2. In a keying system, a plurality of relay stages each comprising a device having a control grid and cathode, a source of potential and a resistance connected in a series circuit, means connecting points on said resistance between the cathode and control grid 0! two of said devices, a first tube having its impedance connected between points on said resistance to shunt a portion of said resistance connected between the control grld'and cathode of one of said devices, a second tube having its points on said resistance to shunt a portion of said resistance connected between the control grid and cathode of the other of said devices, an impedance yariable at signal frequency in shunt to said series circuit, said variable impedance serving to vary the potential drop in said resistance and thereby vary the bias between the control grids and cathodes of said devices to vary the conductivity thereof between a high and low value and connections between the control grids of said first and second tubes and said resistance at points on said resistance which bias said additional tubesby potentials such that the impedances thereof are low when said devices are highly conductive.

3. A keying system-as recitedin claim 2 wherein said variable impedance comprises a keying tube having an anode, a cathode, and a control grid, means connecting the anode and cathode .of said keying tube with said series circuit, and means connecting the control grid of said keying tube to a source of variable potential.

ductive,

HAROLD JOSEPH HUGHES WASSEIL.

impedance connected between 

